This invention relates to methods for optically measuring the position of marks on print media.
It is often necessary to detect and measure marks on media made with ink or any other marking system, using an optical sensor with only its light intensity response and spatial response of dimension zero (such as a radiometer or the like). With the freedom of movement in one or more axis, some spatial information (position) can be extracted from the media.
When an inkjet printer (small or large format) has more than one pen, errors can be produced from a variety of different origins. These errors include, for example, directional errors in the printheads which can produce errors in the paper and pen axes, i.e. the movement of the printhead (Odd/Even scan axis directionality (SAD) errors), and the movement of the media, paper axis directionality (PAD) and swath height errors (SHE). The PAD errors are measured in a direction orthogonal to the SAD error measurement. If the nozzles firing on a pen were perfectly directed, the height of the area printed by the height) should be equal to the height of the nozzles. If there are errors in the paper axis direction, the actual swath height is different from the nominal swath height. The difference is the swath height error (SHE).
Another type of error, when printing in bi-directional modes, is drop placement errors due to the firing signal timing (xe2x80x9cBidirxe2x80x9d error). Drop placement errors can also be due to the printhead-to-printhead distance in both axes (Color to Color SAD and Color to Color PAD). When printing with different modes at different velocities, drop placement errors can occur due to the physics of the drop flight (Bidir). Within the pen, different directional errors from nozzle to nozzle creates error in the position of the dot on the paper (intra-pen SAD). Tolerances and variations of the mechanical parts of the printer can produce drop placement error when the pens are printing. One such error results from bowing developing on the penholder support rod or rail in a large format printer with a long swath distance. This error can provide different pen to paper spacing at different carriage positions.
To obtain high print quality, all of those errors need to be corrected. In the past, the way to do this was create some type of pattern for every defect, measure it and calculate the factor to apply to the electronics or firmware to correct the defect.
An optical sensor for detecting the position of marks on a medium is described, wherein relative motion is provided between the optical sensor and the medium during an optical sensing operation. The optical sensor produces an electrical sensor signal, and has a field of view at the media in a direction of the relative movement. In accordance with an aspect of the invention, the marks have a nominal dimension in the direction of the relative movement, and the field of view is larger than the nominal dimension. This produces a sensor signal with a clear and relatively sharp peak in response to scanning the media mark.
In accordance with another aspect of the invention, a method of sensing directional aberrations among ink-jet nozzles of a plurality of printheads mounted on a scanning carriage is described. The method comprises:
printing an alignment pattern on a print medium, the alignment pattern comprising a set of target marks printed using a subset of the nozzles on each of the plurality of printheads, and a set of monochrome marks printed using a subset of the nozzles from only one of the printheads, each of the monochrome marks positioned in a nominal position with respect to a corresponding plurality of the target marks; and
optically scanning the alignment pattern to determine measurements of relative positions between the monochrome marks and corresponding target marks.
In one embodiment, the target marks include an elongated first mark disposed in a scan direction of scanning movement of the scanning carriage, and an elongated second mark disposed in a media moving direction transverse to the scan direction. In another embodiment, the target marks are patches of similar size to the monochrome marks.
Techniques are described for determining the position of the marks using a linear fitting algorithm.